Fuel cell stack with circuit

ABSTRACT

In a direct-methanol fuel cell stack with an at least partial circulation ( 7, 9 ) of the fuel, according to the invention a means is arranged in this circulation for reducing the concentration of an undesired byproduct. This means is comprised especially of a further electrochemical cell ( 8 ) to which electric current is applied and which at least partly transforms the byproducts by an electrochemical reduction reaction again to methanol and water. Since the concentration of byproducts is small, the loss in electric current required for the additional reduction reaction has no noticeable effect on the efficiency of the fuel cell stack.

[0001] The invention relates to a fuel cell stack in which the fuel is at least in part fed in a circulation.

STATE OF THE ART

[0002] A fuel cell has a cathode, electrolyte and an anode. In a hydrogen fuel cell, the cathode is supplied with an oxidizing agent, for example in the form of air, and the anode is supplied with a fuel, for example in the form of hydrogen.

[0003] Multiple fuel cells are as a rule connected together electrically and mechanically by connecting elements for producing greater electrical outputs. By means of bipolar plates fuel cells are stacked one above another and connected electrically in series. These arrangements are termed fuel cell stacks. They are comprised of the bipolar plates and the electrode-electrolyte units.

[0004] From the literature, fuel cells and fuel cell stacks are known which apart from the supply and discharge passages for the operating media, have available an additional circulating path for the fuel (anode side circulation). This means that the fuel fed to the anode compartment, which usually is not completely electrochemically converted, is at least partly recycled. As a result the fuel can be more effectively utilized. For carrying out the recycling, a recirculating tank with a recirculating pump are provided.

[0005] For a direct-methanol fuel cell, the fuel is supplied in the form of a methanol-water mixture. With an anode side recycling of the fuel, the depleted, that is partly converted, methanol-water mixture, is withdrawn from the anode compartment. A part of this mixture is discharged. The remaining part is restored to the starting concentration by the addition of pure methanol. The thus formed methanol-water mixture can then be fed again to the fuel cells.

[0006] It has been found that with a direct-methanol fuel cell, apart from the desired electrochemical conversion of the fuel at the anode, there are undesirable side reactions. The byproducts resulting from these side reactions are typically discharged with the nonconverted fuel from the fuel cell. With an anode side circulation, however, these byproducts which are formed tend to become concentrated in the recycle with time. Formaldehyde and formic acid especially are the typical byproducts which arise. As a result of the concentration, these byproducts have a negative effect on the durability of the fuel cell and usually increase the requirements for the quality of the materials from which the fuel cell is made. This gives rise especially to a reduced economy of the system. Simultaneously the potential which is developed at the anode is usually negatively influenced by the presence of byproducts.

OBJECT AND SOLUTION

[0007] The object of the invention is to provide a fuel cell stack with a recirculation in which the above-mentioned drawback can be avoided. Further it is an object of the invention to provide a method of operating such a fuel cell stack.

[0008] The objects of the invention are attained with a fuel cell stack with the totality of the features of the main claim as well as with a method according to the auxiliary claim. Advantageous embodiments are to be derived from the subordinate claims respectively dependent thereon.

SUBJECT OF THE INVENTION

[0009] The invention relates to a methanol fuel cell stack with a recirculation of the fuel. The fuel cell stack according to the invention has at least one means for reducing the concentration of at least one of the byproducts in the fuel cell recycle.

[0010] With a direct-methanol fuel cell stack, a methanol-water mixture is used as the fuel. By a recycling of the fuel it is to be understood that the fuel which is not completely converted, after leaving the anode compartments is at least partly returned to a fuel cell of the fuel cell stack. Since the concentration of methanol is reduced by the electrochemical conversion at the anode, advantageously by addition of methanol, the starting concentration of methanol is restored before the fuel is fed to a further fuel cell.

[0011] As byproducts all chemical compounds are to be considered which are not given directly by the following conversion reaction:

CH₃OH+H₂O→CO₂+6H⁺+6e³¹

[0012] This means all compounds aside from water, methanol and CO₂ count in the scope of this invention as the byproducts. Especially the compounds formaldehyde (CHOH) and formic acid (CHOOH) belong to this class. During the recirculation the concentrations of these byproducts in the fuel recycle increases. A means for reducing the concentration of at least one of the byproducts is, for example, a further electrochemical cell or also an adsorber, an absorber or also a filter.

[0013] In an advantageous configuration according to claim 2, the fuel cell stack has a device which enables at least one of the byproducts to be electrochemically transformed. Especially, the fuel cell stack according to the invention has a further electrochemical cell according to claim 3 as the device for electrochemically converting at least one of the byproducts. Advantageously this is an electrochemical cell which is constructed like a direct-methanol fuel cell. This is arranged in a suitable manner in the fuel circulation. Since the circulation of byproducts is relatively small by comparison to the fuel, the electrochemical cell for converting the byproduct usually can be significantly smaller in construction than the fuel cells of the stack used for electric current generation.

[0014] Within the framework of the invention it has been found that a reversal of the anodic reaction which takes place in a fuel cell can be used to reduce byproducts which can arise in a fuel cell stack with fuel recirculation in a regular manner. It is thus possible to hold the concentration of byproducts during the operation of the fuel cell to a low level and prevent a concentration of them.

[0015] The method of the invention for operating a direct methanol fuel cell stack with at least partial circulation of the fuel encompasses the following steps:

[0016] Fuel which has at least one byproduct, is at least partly fed to a further electrochemical cell.

[0017] In this further electrochemical cell at least one product is at least partly electrochemically converted by a reducing reaction.

[0018] Thus the only partly converted fuel which contains undesired byproducts can be fed in a suitable manner to a device in which an electrochemical conversion of at least one of the byproducts is carried out.

[0019] Advantageously according to claim 6, as a further electrochemical cell, a direct-methanol fuel cell is used. As a result, by the application of an appropriate electric current to this electrochemical cell, the anode reaction of a current generating direct methanol fuel cell is carried out in reverse (reduction reaction), and at least one byproduct is electrochemically transformed thereby.

[0020] According to claim 7 in this further electrochemical cell methanol is produced. The methanol is produced by the electro-chemical conversion of one or more byproducts together with water as the reverse of the anode reaction in a fuel cell stack. Since the concentration of the byproducts by comparison with fuel concentration in the fuel circulation is low, the fuel cell used for transforming the byproduct can be significantly smaller than the fuel cells which are used for electric current generation in the stack.

[0021] The current which is applied to the additional fuel cell serves for the electrochemical transformation of at least one byproduct. The electric current required for this purpose is small compared with the electric current generated in the fuel cell stack so that the effectiveness of the entire system is not significantly reduced by the additional fuel cell. However, because of concentration of byproducts in the fuel circulation is held low, the potential at the anode is advantageously scarcely affected in a negative manner and the determining influence of the byproduct on the materials of the fuel cell stack is uniformly precluded.

LEGENDS FOR THE FIGURE

[0022]1 Fuel cell stack

[0023]2 Anode compartment

[0024]3 Cathode compartment

[0025]4 Electrolyte

[0026]5 Bipolar plate

[0027]6 Supply channel for fuel

[0028]7 Discharge channel for depleted fuels

[0029]8 Additional electrochemical cell for converting at least one byproduct

[0030]9 Circulating tank

[0031]10 Circulating pump 

1. A direct methanol fuel cell stack with at least two fuel cells, characterized by a circulation for the fuel and a means for reducing the concentration of at least one byproduct in the fuel circulation.
 2. A fuel cell stack according to the preceding claim which has a device for electrochemical conversion of a byproduct as the means for reducing the concentration of byproducts.
 3. A fuel cell stack according to one of the preceding claims with a further electrochemical cell as means for reducing the concentration of byproducts.
 4. A fuel cell stack according to a preceding claim with an electrochemical cell which corresponds in construction to a direct methanol fuel cell as a means for reducing the concentration of byproducts.
 5. A method of operating a direct methanol fuel cell with at least partial circulation of the fuel with the steps of: feeding fuel together with at least one byproduct at least partly to a further electrochemical cell; in this electrochemical cell converting at least one byproduct in a reduction reaction at least partly electrochemically.
 6. The method according to the preceding claims in which an electrochemical cell which corresponds in construction to a direct methanol fuel cell, is used as a further electrochemical cell.
 7. The method according to one of the preceding claims 5 to 6 in which the electrochemical conversion reaction in the further electrochemical cell produces methanol. 